Active RFID tags include fully active (battery operated) RFID tags and semi-passive (battery-assisted passive) RFID tags. Both of these are discussed in general terms herein, and configurations using power storage mechanisms can often include either device. However, either of these devices can be referred to as a power-enhanced RFID tag, in which stored power from the battery or other storage device is used to enhance RFID operations, provide more efficient power usage, increase tag lifespan, and enhance overall the power-related operations for the RFID tag.
The supplemental power allows active or semi-passive (battery assisted passive) RFID devices be much more effective and versatile compared with purely passive RFID devices. They also have exceptional receive sensitivity when operating in a power-assisted mode versus passive RFID devices. In addition, active or semi-passive (battery assisted passive) RFID devices can perform additional functions under their own power even when not being actively interrogated including collecting sensor data, activating external actuators, and running complex software for cryptography or other purposes. However, active or semi-passive (battery assisted passive) RFID devices are limited in performing their enhanced communications, carrying out supplemental functions and running software due to the limited life of their battery or supplemental power supply.
The battery life limitation for conventional powered, active or semi-passive (battery assisted passive) RFID devices is inherent due to their reliance on standard, non-rechargeable batteries as their supplemental power source. These devices have a limited life based on the initial charge of their batteries and the rate at which they draw power. Typical RFID applications such as globally monitoring the location of 40-foot ocean-going containers are severely constrained, because it is very difficult to manage and service RFID tags in use prior to depletion of the power supply. Larger capacity batteries including using multiple batteries with RFID devices have been tried, but once these batteries begin to discharge, they deteriorate relatively quickly and reach their lifetime limits quickly; albeit, slightly longer than with regular capacity batteries.
There has been much discussion and some demonstration of “harvesting” energy from existing RF fields, but generally these fields are very weak and are insufficient to recharge batteries. Further, conventional rechargeable batteries require a significant potential difference in order to reverse the chemical reactions used to store the electrical energy. There have also been attempts to use super-capacitors to store power instead of rechargeable batteries and, thus, avoid the required large potential difference for recharging batteries. However, capacitors and even super-capacitors are much less effective at storing charge for extended periods compared with batteries.